1. Field of the Invention
This invention relates generally to adhesive tapes useful for processing semiconductor wafers and, more specifically, to tapes having nonpressure sensitive adhesive compositions that include a copolymer of acrylic-functional monomers. These tapes are useful in the grinding and/or dicing of semiconductor wafers.
2. Description of the Related Art
Semiconductor integrated circuit (IC) chips are commonly used in electronic components, whether for sophisticated industrial machinery, automobiles or everyday household appliances. The production of semiconductor IC chips begins with the manufacture of semiconductor wafers containing many semiconductor elements. Ultimately, the wafer is sawn or diced into individual semiconductor elements (called die), each element becoming a semiconductor IC chip.
Typically, a semiconductor wafer is prepared by slicing or sawing a single, high purity silicon ingot into thin circular wafers about 500 microns (.mu.m) to about 1000 .mu.m thick. A wafer may be doped to alter its electrical properties. Electronic circuitry is then applied to the front side of the wafer, usually by photolithography. Separation lines are also photolithographed onto the wafer to provide saw marks for eventual dicing of the wafer into individual semiconductor IC chips.
Wafer diameters were traditionally about 3 inches (7.6 cm) to about 4 inches (10.2 cm). However, as individual IC chips have become larger, the typical wafer diameter has increased to about 5 inches (12.7 cm) to about 8 inches (20.3 cm) so as to permit more die to be formed from a single wafer. It is expected that wafer diameters will eventually expand to about 12 inches (30.5 cm) to about 16 inches (40.6 cm), and perhaps to even larger sizes.
To protect the delicate electronic circuitry from atmospheric contamination by dust, moisture, airborne corrosive acids and the like, the front side of the wafer is provided with a passivation or protective layer which may be an inorganic material such as silicon oxynitride or an organic material such as polyimide.
To facilitate the manufacture of electronic components, it is desirable to reduce the thickness of the wafers (and hence the thickness of the semiconductor IC chips formed therefrom). A common process involves holding the front side of the wafer against a vacuum table while grinding the backside of the wafer to a thickness of about 200 .mu.m to about 500 .mu.m in the presence of a water spray to remove the grinding debris. However, the wafers are inherently fragile and are susceptible to breaking during the grinding process, a problem which is enhanced as the wafer diameter becomes larger. Moreover, the front side of the wafer is held against the vacuum table, which could abrade the passivation/protective layer and the underlying circuitry. Consequently, there is a need to protect the wafer (especially the front side) from breakage, contamination, and abrasion.
Early approaches to this problem used a layer of paraffin wax over the front side of the wafer, with the wax being eventually removed by a solvent wash. The deficiencies of this approach are described in U.S. Pat. No. 4,853,286 (Narimatsu et al.). In other approaches, a photoresist coating was spin-coated onto the front side of the wafer, but this did not always eliminate wafer breakage.
Historically, adhesive products have been employed to protect the front side of the wafer. Sometimes the tapes are used alone and sometimes they are used in conjunction with a photoresist coating to provide a surface to which the tape can stick and to protect the passivation/protective layer from staining and/or delamination by the tape. However, according to the technical literature, adhesive tapes have not provided a complete solution to the wafer protection problem. The previously mentioned U.S. Pat. No. 4,853,286 indicates that wafer breakage still occurs and that the adhesive surface accumulates dust that can contaminate the wafer. Also, there can be difficulty in subsequently removing the tape if it has high initial adhesion to the wafer or if the adhesion increases from the time that the tape is applied to the wafer until it is removed.
Various adhesive tapes that are reportedly useful in semiconductor wafer backside grinding operations (sometimes referred to herein as "wafer grinding") have been described. For example, the aforementioned U.S. Pat. No. 4,853,286 discloses a wafer processing film that is used in the grinding of wafers to prevent breakage. The film includes a base film, a layer of a commercially available, common adhesive (such as an acrylic ester, urethane, or synthetic rubber adhesive) that can preferably include a nonionic surface active agent and an ethylene glycol derivative, and an optional supporting film laminated to the nonadhesive side of the base film. U.S. Pat. No. 5,126,178 (Takemura et al.) describes a wafer processing film that includes a base film with a pressure sensitive adhesive on one side (which is protected by a removable release film), and a phosphoric acid-based surfactant on the backside. The pressure sensitive adhesive can be acrylic-based, vinyl-based, or rubber-based, although an aqueous emulsion type pressure sensitive adhesive is preferred.
European Patent Publication No. 0 530 729 discloses a pressure sensitive adhesive tape used in grinding the backside of a semiconductor wafer. The pressure sensitive adhesive, which reportedly has a small initial adhesion and shows no adhesion strength increase with time, comprises an aqueous acrylic resin emulsion adhesive, a nonionic surfactant, an epoxy type and/or an aziridine type crosslinking agent, and a water soluble organic compound.
However, there still remains a need for an adhesive tape that has even greater utility in semiconductor wafer grinding processes. Preferably, such tapes will possess several desirable properties. For example, the tape should preferably quickly provide sufficient initial adhesion to surfaces such as silicon, polyimide, silicon oxynitride, and other integrated circuit coatings such that the semiconductor wafers will readily survive post-processing steps yet be easily removed when required. Preferably, a single tape will provide acceptable initial adhesion to each of these surfaces so as to eliminate the need for storing different tapes for different surfaces. However, the final adhesion should not be so high that removing the tape breaks or fractures a larger number of wafers than is permitted under conventional industry standards (typically about one wafer or less per one thousand), or leaves adhesive residue that could impair subsequent processing of the wafer. Many tapes used in the semiconductor industry require the application of heat to the wafer at the time of removal of the tape to prevent the wafer from being broken. It would be desirable if no such heating step were required.
It would also be desirable if the initial and final adhesion properties of the tape were preferably maintained over several days and, more preferably, over several weeks of storage. That is, preferably, there should be no process or material-limiting increase in adhesion over time (sometimes referred to as adhesion build), a problem associated with certain adhesives. Similarly and preferably, there should be no other significant change in adhesion over time, as could occur if fugitive or migratory surfactants and other mobile components in the adhesive migrate to the adhesive bond line so as to form a weak boundary layer. An adhesive that generally maintains its initial and final adhesion properties during storage would not only provide tapes having long shelf lives, but would also eliminate the need to carry out the grinding process shortly after taping the semiconductor wafers.
Another desirable attribute would be the ability to remove the adhesive tape without substantial staining, which refers to a change in the optical density of the semiconductor wafer that is detected when the semiconductor wafer is viewed under a microscope. This typically results from microscopic amounts of adhesive residue being left on the passivation layer, although it could also result from partial removal of the passivation layer. It would also be desirable if the adhesive were generally insensitive to water so as to prevent the tape from being loosened by the water spray used during grinding. Furthermore, it would be desirable for the adhesive to have a generally flat profile so that unevenness in grinding can be avoided. Also, it would be desirable if the tape did not contribute significantly to the formation of edge debris during trimming of excess tape upon application to a wafer.
Following wafer grinding, there are typically several intermediate manufacturing steps before the semiconductor wafers are sawed or diced into individual semiconductor IC chips. Wafer dicing is conventionally carried out by attaching the ground backside of the wafer to the adhesive surface of a tape (often called a dicing tape), securing the taped wafer to a vacuum table to restrain it against movement, and using a water-sprayed rotary diamond saw to cut along the saw marks previously photolithographed onto the semiconductor wafer. The individual semiconductor IC chips are then removed from the dicing tape. This operation is usually facilitated by a needle or probe that pushes up against the backing of the dicing tape in the area of the IC chip while a vacuum pick-up simultaneously grasps the top of the IC chip to remove it from the dicing tape. The removed IC chips may then be further processed immediately, or they may be stored for later assembly into a finished article.
Prior technical publications describe various difficulties encountered when using adhesive dicing tapes. For example, European Patent Publication No. 0 157 508 discusses the need for an adhesion level sufficient to permit the semiconductor wafers to stick to the tape but without being so high as to impede removal of the diced IC chips.
Adhesive products, particularly pressure sensitive adhesive tapes, for use in wafer dicing operations have been previously described. For example, European Patent Publication No. 0 588 180 describes a dicing tape that includes a radiation transparent film and a radiation crosslinkable pressure sensitive adhesive that contains a copolymerized radiation-initiator. The pressure sensitive adhesive may be synthesized from (meth)acrylic acid, (meth)acrylic ester, vinyl acetate, or various vinyl alkyl ether monomers.
Japanese Kokai Patent Application No. 62-121781 describes a semiconductor wafer dicing film in which a conventional pressure sensitive adhesive is applied to a butene type polymer film. Japanese Kokai Patent Application No. 5-230426 discloses an adhesive tape for wafer dicing that includes an adhesive layer (especially an acrylic type adhesive) on a base film made of a rubber-like elastic material.
The previously referred to European Patent Publication No. 0 157 508 describes a thin adhesive sheet which is used to protect a semiconductor wafer during the polishing step or to fix the wafer when cutting and separating the semiconductor wafer into the IC element chips. The adhesive sheet includes a light-permeable support and a pressure sensitive adhesive that can cure by light irradiation to form a three-dimensional network structure. The adhesive comprises a mixture of a rubber- or acryl-based pressure sensitive adhesive, a photopolymerizable compound, and a photopolymerization initiator.
However, there still remains a need for an adhesive tape that has even greater utility in semiconductor wafer dicing processes. Preferably, such tapes will possess several desirable properties. For example, the tape should preferably provide sufficient initial adhesion to the silicon wafer (as well as other surfaces to which the tape may need to adhere such as gold plating or stainless steel machine parts) such that the resulting semiconductor IC chips will not come loose during wafer dicing. However, the final adhesion should not be so high that the semiconductor IC chips fracture or break upon removal from the tape. It would also be desirable if the initial and final adhesion properties of the tape were preferably maintained over several days and, more preferably, over several weeks of storage since several days or weeks may pass between the time that the semiconductor wafers are attached to the dicing tape and the time that the resulting semiconductor IC chips are removed from the tape after dicing. If the adhesion increases substantially over time, it may be necessary to remove the diced semiconductor IC chips from the tape and store them, unless they will be used immediately after dicing.
The semiconductor IC chip should also preferably remove cleanly from the adhesive tape so as to not leave adhesive residue that could interfere with subsequent processing such as soldering and packaging of the IC chips. It would also be advantageous if the adhesive from the tape did not stick to the saw blade as this could require periodically interrupting the wafer dicing operation to clean the accumulated adhesive residue from the blade so as to prevent contamination of the semiconductor IC chips. Also, if the adhesive does not significantly stick to the saw blade, then it could be possible to employ a thicker layer of adhesive, which could be advantageous in preventing the saw blade from inadvertently cutting into the tape backing. If the tape backing is partially cut into during wafer dicing, the tape could be weakened and break prematurely during subsequent processing.
It would also be desirable if the adhesive were generally insensitive to water so as to prevent too many chips from being loosened by the water spray used in the dicing process. Furthermore, it would be desirable if the tape did not contribute significantly to the formation of edge debris when trimmed prior to a wafer dicing operation.
Finally, it would be advantageous to have a single adhesive tape that could be used in both wafer grinding and wafer dicing operations so as to eliminate the need for storing different tapes for different processes. The present invention provides wafer processing adhesives and tapes that have one or more of these desirable properties and can be used in grinding and/or dicing operations during the manufacture of semiconductor wafers.